The invention relates to a vehicle transmission shift control system, and more particularly, to a clutch slip-based powershift transmission shift control system.
Modern vehicle powershift transmissions, such as the AG250 powershift transmission manufactured by Funk Manufacturing, or such as described in U.S. Pat. No. 5,557,978, issued Sep. 24, 1996 to McAskill, and assigned to the assignee of this Application, contain multiple electronically controlled, hydraulically actuated wet clutches. Such clutches direct the flow of power through the transmission, and the control of such clutches permits selection of different gear ratios. Conventional shifting control of these types of transmissions involve fixed timing of the clutch solenoid valve current values based solely on a signal representing engine load recorded at the beginning of a shift. Specifically, each clutch involved during a particular shift has its own engagement or disengagement function to perform according to this load signal. For this type of control scheme to function properly, each clutch engagement and disengagement path for each shift must be mapped out according to the load signal. This can result in many iterations, because often a certain set of clutch paths has to be resolved before another can be resolved. To add to the confusion, the clutches will often interact with one another (especially in a dynamic slip condition) thus making it very difficult and time consuming to find the optimum engagement or disengagement path for every clutch in a particular shift across the engine load spectrum.
Using the engine load signal itself to determine clutch operation has a few shortcomings. First, the engine load signal varies significantly. If clutch engagement and disengagement paths are based solely on an engine load signal, inconsistency in the feel of the shifting can be expected, because clutch engagement pressure steps will change accordingly. Secondly, using only the engine load signal to determine clutch engagement and disengagement paths can be misleading. For example, in agricultural tractor applications, there are situations where engine load does not correspond to actual load at the particular output clutch in the transmission. In such situations, the engine""s primary use is to power auxiliary or external mechanisms through either a power-take-off (PTO) or a hydraulic pump. Regardless, the transmission output clutch does not see this load. The engine load signal to the transmission continues to supply a signal representing the load, and such a load signal cannot distinguish between an auxiliary load and an engine load relating to transmission load. Therefore, if the vehicle itself is lightly loaded by draft force yet the engine is heavily loaded by an auxiliary function, then clutch operation will be what would be appropriate for a heavy draft load condition, resulting in a very aggressive shift and transmission of the shift shock to the operator. This will be perceived by an operator as a bad shift.
Accordingly, an object of this invention is to provide a powershift transmission control system which produces smooth shifts regardless of what particular shift is commanded.
Another object of this invention is to provide such a powershift transmission control system which controls the order in which clutch pairs are swapped depending upon whether the input speed of the transmission will speed up or slow down.
Another object of this invention is to provide such a powershift transmission control system wherein the last clutches to be swapped are swapped when the monitored speed ratio matches a stored speed ratio.
These and other objects are achieved by the present invention for a method of controlling, in response to a shift command, a commanded shift of a powershift transmission of an engine-driven vehicle. The transmission has an input shaft, an input section and fluid pressure operated clutches for controlling flow of torque through the transmission. The transmission includes output clutches and speed clutches between the output clutches and the input-shaft. According to the present invention, the method includes monitoring a speed of a first component of the transmission, monitoring a speed of a second component of the transmission, the first and second components being selected according to the commanded shift and monitoring a speed ratio of the speed of the first component to the speed of the second component. If the input section will speed up during the commanded shift, the speed clutches are swapped, and then the output clutches are swapped when the monitored speed ratio matches a stored speed ratio. Also, input speed of an input shaft of the transmission is monitored, and the pressure of the off-going output clutch is controlled to limit the deceleration of the input shaft speed to desired limits. If the input section will slow down during the commanded shift, the output clutches are swapped, and then the speed clutches are swapped when a monitored speed ratio matches a stored speed ratio. If a particular shift requires swapping of only speed clutches, then a speed ratio is monitored for the pair of the speed clutches to be swapped. If the input section will speed up or slow down during the commanded shift, the pair of speed clutches are swapped, and the monitored speed ratio is compared to a predetermined stored speed ratio value to determine when swapping of the speed clutches is completed, and after the speed clutches are swapped, one of the output clutches is fully pressurized to complete the shift.